For a mobile station connectable to multiple radio communication systems, such as a 3G (Wideband-CDMA) system (hereinafter, 3G) and a Long Term Evolution system (hereinafter, LTE), various methods (Inter-RAT mobility) of controlling the mobile station between radio communication systems have been specified.
For example, a mobile station (UE) being in connection with an LTE using a PS bearer, which is a packet switched-type communication bearer, can perform fallback (CSFB) to a 3G using a CS bearer, which is a circuit switched type communication bearer.
In this manner, the CSFB is a control method in which, when an LTE does not support a circuit switched (CS) call, the UE is transitioned to a radio communication system supporting the CS call, such as UTRAN (or GERAN of GSM (registered trademark)) in the 3G, so that the CS call is connected with the UE.
In a method of transitioning UE to a radio communication system supporting the CS call, procedures such as PS HANDOVER, Redirection with/without System Information Block, Redirection, and CELL CHANGE ORDER (see, Non-patent document 1) are specified. Specifically, an eNB (radio base station) determines which one of the above-described procedures is executed based on the capability of the UE.
Here, when the UE is in connection with the LTE using the PS bearer, i.e., when the UE is in connection with a PS call, it is a general practice that the PS HANDOVER procedure is preferentially executed for continuously utilizing the PS bearer also after the UE has transited to another radio communication system.